A good quality screen is essential for a pleasant experience. This is my take on each one of these devices.
Dingoo Digital A320
Size: 2.8 Inches
Resolution: 320 x 240
While the Dingoo has great battery life the screen quality isn't the best. Since the left and right are different my eyes don't get the exact images, which can be a bit annoying. The best viewing angle is looking at the device from slightly top and front.
Size: 4.3 Inches
Resolution: 430 x 272
This is better than the Dingoo Digital. Since the left and right sides look the same the experience is better. The colors don't look as good as the JXD5000.
Size: 5 Inches
Resolution: 800 x 480
Overall very good. The colors on the JXD500 are sharper than the JXD F3000. When dark colors are shown you can notice a bit of edge bleeding on the lower left side of the screen. But overall the bigger screen on the JXD5000 is better than the other two.